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Cathodic and Anodic Protection
THOMAS THANGADURAI K
“The reduction or prevention of corrosion by making metallic
structure as cathode in the electrolytic cell is called cathodic
Since there will not be any anodic area on the metal, corrosion
does not occur.
There are two methods of applying cathodic protection to
(i) Sacrificial anodic protection (galvanic protection)
(ii) Impressed current cathodic protection
Sacrificial anodic protection method
In this method, Metallic structure is connected to more anodic
Hence the corrosion is concentrated at the active metal only.
And the active metal gets corroded slowly.
The parent structure (cathodic part) is thus protected.
The more active metal so employed is called sacrificial anode.
Sacrificial anodes are commonly as Mg, Zn, Al and their alloys.
Protection of underground pipeline with
a magnesium anode
Applications of sacrificial method
• Protection of buried pipelines.
• Underground cables.
• Marine structures.
• Ship hulls.
• Water tanks.
Cathodic protection of a domestic hot water tank
using a sacrificial anode.
Impressed current cathodic protection method
The sufficient D.C current is given to underground tank.
The negative terminal is connected to tank.
And the positive terminal is connected to inert anode.
When the power supply is ON, it converts corroding metal from
anode to cathode.
Hence the corrosion can be prevented.
Cathodic protection of an underground tank using
Applications of impressed current method
• it is useful for large structures
• Applied to open water box coolers
• Water tanks
• Buried water and oil pipelines
• Transmission line towers.
Stray currents resulting from cathodic protection
Anodic protection (AP) is a technique to control
the corrosion of a metal surface by making it the anode of
an electrochemical cell and controlling the electrode potential in
a zone where the metal is passive.
The application of anodic current to the structure
decrease the rate of hydrogen evolution.
To anodically protect the structure, potentiostat is required.
Potentiostat is an electronic device that maintains a metal at a
The use of potentiostat is to shift corrosion potential into passive
potential so that the corrosion of the metal is stopped.
The potential range depends upon the relationship between the
metal and the environment.
Applications of anodic protection
Applicable in extremely corrosive environments.
Low current requirement.
Acid coolers in dilute sulphuric acid plants.
Storage tanks for sulphuric tanks
Chromium in contact with hydrofluoric acid.